DESCRIPTION: The overall goal of this research is to determine how gonadal steroids modify the brain to produce masculine behavior. To achieve this goal, testosterones organizing effects during development and subsequent activational effects in adulthood will be examined. Previous reports correlate masculine behavior with a sexually dimorphic area (SDA) in the gerbil brain. Although SDAs and sexually dimorphic nuclei have been reported in the brains of many vertebrates, including humans, the gerbil provides a model in which neural substrate has been directly linked with behavioral output. The gerbil SDA is a complex of hormone-accumulating cells groups found within the preoptic area. Published findings indicate that the SDA and subgroups within the SDA, e.g., a subgroup found primarily in males, the SDA pars compacta (SDApc), appear to be involved in different aspects of masculine behavior. The volume of the SDA correlates with copulatory rate while the volume of the SDApc correlates with the males ability to achieve intromission. The lateral portions of the SDA appear important for scent marking behavior, a form of olfactory communication. During the period of requested support, this gerbil model will be more fully developed by determining: 1) whether the time course of development of gonadal steroid receptors within the SDA corresponds to the time course of the differentiation of the SDA and copulatory behavior by investigating the development of estrogen and androgen receptors in the SDA, 2) the time course of secretion of testicular steroids during the perinatal period of gerbil development, 3) if the maintenance of the SDApc and differentiation of the SDA in normal males and androgenized females as compared to normal females occurs via differential cell death within the SDA and SDApc, and 4) the role of prenatal steroids in the development of masculine behavior and two of its underlying substrates, the SDA and SDApc. Using this model the proposed research will delineate the simultaneous differentiation of the SDA, SDApc, scent marking, and copulatory behavior. Further, it will provide evidence for the mechanism of the differentiation of behavior by identifying the neuroanatomical changes in the SDA, an area controlling sociosexual behavior.